One of the most important aspects of human immunodeficiency virus type-1 (HIV-1) env variability concerns its effect on immune recognition, which has profound implications for vaccine design. We are characterizing env determinants which are important for an effective immune response and studying how variability affects such a response. One approach involves transmitting and culturing molecularly cloned HIV-1 strains in the presence of natural antisera containing high titers of group- or class-specific neutralizing antibodies. Variants can be selected which are resistant to neutralization by the selecting antiserum. We have characterized one such variant as well as the nature of the altered site, and are currently studying a second. Other studies involve viral isolates obtained from defined risk groups in Zaire. This includes virus isolation, determination of env sequences, and analyses of appropriate sera for reactivity to peptides representing different parts of the envelope and ability to neutralize different wild-type and envelope chimeric viruses. We have established 14 Zairian isolates and sequenced portions of the gpl2O of multiple clones from each isolate, including the PB1 neutralization loop. We have made peptides from these loop sequences, and are currently raising antisera against them. We have begun to define the reactivity patterns of the homologous sera against these peptides, as well as the ability of the sera to neutralize viruses containing different loops. We have also developed an infectious (IIIB) clone into which the PBl loop of any virus is easily substituted, and have made (IIIB) chimeras with the (RF) and (BA-L) loops.